1. Field of the Invention
The present invention relates generally to sensing systems for the detection of worn and broken cutting tools in machine tool centers and, in particular, to a new and novel method and apparatus for measuring the cutting forces for such a system.
2. Description of the Prior Art
Modern high-speed machine tools are capable of significant improvements in productivity. However, in order to achieve these gains in productivity, it is necessary to replace human operators with computer numerical control (CNC) systems. Unlike a human operator, a CNC control system does not inherently have means for detection of worn and broken cutting tools. Furthermore, because such systems are designed to remove tremendous amounts of metal in a short period of time, extremely high cutting speeds must be used. As a result, it has become critical that worn and broken cutting tools be detected in order to stop the machine tool before it can inflict considerable damage to itself or its workpiece.
Changes in the tool condition, including wear or breakage, are accompanied by characteristic changes in the cutting forces from "normal" cutting force signals. Tool condition sensor systems use advanced algorithms to detect these characteristic changes. Such systems utilize separate, specialized algorithms to detect wear, breakage and collision, and provide separate outputs to the CNC control for each occurrence.
Tool breakage is a sudden, catastrophic event characterized by distinct changes in force signals. These changes are not a signature per se but are a characteristic sequence of events (i.e. a fingerprint). Thus, as long as the sensor system is capable of detecting the event, it is not necessary that the accuracy of the force measurement be very high. On the other hand, the detection of tool wear requires transducers which have very little hysteresis, very good linearity, and which are extremely stable with respect to movement of the machine slide or changes over time. Thus, the weak link in present day tool condition sensor systems is obtaining good transducer output representative of the cutting forces.
Typically, to measure cutting forces, sensor systems utilize one or more load cells. In machine tool applications, the load at the tool tip can reach 10,000 pounds and, because of lever arm effects, the load on a load cell can be even higher. Thus, it has been necessary to specially modify load cell installation in some manner in order to at least indirectly measure the cutting forces at the tool tip while protecting the load cell from overload and still working within the constraints of the machine structure.
One such system which attempts to meet this criteria is disclosed in German Patent DE No. 3440670 to Kluft. Kluft attempts to measure three-axis cutting tool loads by utilizing a single axis load cell or cells and calculating the three-axis load through moment arm calculations. This method of measuring cutting forces is obviously open to the introduction of significant error. Moreover, because of the dependency on moment arms, the force transducer plate cannot be placed in or even near the plane of the cutting tool. Furthermore, because of variations in moment arm due to changes in the tool tip location for different tools, the calibration of these devices will change with each new tool tip location. Thus, application is limited.
Finally, while Kluft did address load cell overload, it does not address the problems of hysteresis, linearity, and stability which are most common in plate-type force transducers.
Thus, it has become desirable to develop a sensor system for a tool condition monitoring system which must not be constantly recalibrated while, at the same time, providing a signal indicative of the cutting forces which has very little hysteresis, very good linearity and is extremely stable.